Temper rolling is performed on a steel strip by skinpass rolling, for example, at a reduction of 1% or less using a temper rolling mill. By performing this temper rolling, a steel strip is equally elongated, and the shape thereof is corrected, so that a predetermined flatness can be obtained. In addition, by the temper rolling, for example, mechanical properties, such as the yield elongation, the tensile strength, and the elongation, and surface roughness of a steel strip can also be improved.
In recent years, concomitant with development of high-value added steel strips, a steel strip made of hard steel, such as so-called high tensile-strength steel or high-carbon steel, has been increasingly in demand. When a steel strip made of hard steel as described above is processed by temper rolling using a temper rolling mill, a high rolling load (rolling burden) is required to impart a necessary elongation percentage to the steel strip. In particular, it has been difficult to impart an elongation percentage to thin hard steel having a thickness of 1.0 mm or less.
In addition, among high tensile-strength steel sheets, a steel sheet manufactured by continuous annealing including a quenching treatment and a tempering treatment has a problem in that the surface shape thereof is deformed, during the quenching treatment, by thermal stress and/or phase transformation of steel microstructure, so that a shape defect is liable to occur. Even when a steel-sheet surface is planarized by cold rolling before annealing, it is difficult to overcome this shape defect of a steel sheet. Accordingly, it is desirable to correct the shape of a steel sheet by temper rolling after annealing. However, in the case of a high tensile strength steel sheet having a tensile strength of 980 MPa or more, when an elongation percentage required for shape correction is imparted thereto, a flow stress is high, and hence a very high rolling load is required.
In particular, for a high tensile-strength steel that requires shape correction, a higher rolling load is required, and hence it is sometime difficult for an existing temper rolling mill to perform the shape correction. Accordingly, the shape correction is actually performed in such a way that after temper rolling is performed, a shape-correction step is additionally performed. However, in this case, concomitant with an increase in number of steps, problems, such as an increase in manufacturing cost and a longer delivery time, occur.
Furthermore, in the situation described above, hard steel having properties that require higher facility performance than that of an existing facility has been introduced, and the number of cases in which correction cannot be performed by an existing temper rolling mill starts to increase; hence, the countermeasures therefor have been strongly desired.
For example, as one of the countermeasures for the above problems, a method may be mentioned in which temper rolling is performed while a high tensile force is applied to a steel strip. By this method, although it is possible to impart a sufficient elongation percentage at a low rolling load, since bridle rolls must be additionally provided, or the number of which must be increased (for example, the number of rolls is increased from two to three) in order to ensure a necessary high tensile force, a large installation space is required, and facility cost is also increased.
As another countermeasure, although a method may also be mentioned in which a temper rolling mill that can impart a high load is manufactured, since a housing capable of withstanding a correction load is required, a large installation space is also required, and facility cost is increased.
In addition, although a method may also be mentioned in which the diameter of each work roll is decreased, since the deflection of the work roll has a serious influence on a steel strip shape, a highly-accurate shape control system in consideration of this influence must be provided. Furthermore, due to a decrease in withstand load of the roll caused by the decrease in diameter thereof, the rolls may even be broken in some cases.
In order to overcome the problems described above, in Japanese Unexamined Patent Application Publication No. 10-5809 (Patent Document 1), a technique has been disclosed in which by performing temper rolling at a predetermined strain rate in a predetermined warm temperature region, a decrease in rolling load is realized, and temper rolling can be performed on hard steel.
In addition, as another problem concomitant with the increase in strength of a steel strip, since a load applied during press forming increases, and a stress between a press die and a steel strip becomes very high, die galling is disadvantageously liable to occur.
In order to improve die galling resistance, although it is believed that the control of surface roughness of a steel sheet may have an effect to a certain extent, the surface roughness that can be imparted to a hard steel sheet by conventional temper rolling is very limited, and another method for imparting surface roughness has also been proposed. For example, in Japanese Unexamined Patent Application Publication No. 2006-7233 (Patent Document 2), rolling is performed using dull rolls provided at a final stand of cold rolling, and the surface roughness is formed in the surface of a steel strip.
However, in the method for performing temper rolling on a steel strip disclosed in Japanese Unexamined Patent Application Publication No. 10-5809 (Patent Document 1), the temperature of every steel strip to be processed by temper rolling must be controlled, and the control is not only complicated, but an apparatus and a system used for the temperature control are also required. In addition, in order to perform warm rolling, when the difference in temperature is generated in a width direction of a steel strip, the flow stress varies in the width direction, and the shape of the steel strip after rolling may be influenced thereby in some cases. Furthermore, when the flatness is significantly improved in the state in which the difference in temperature is present, after the temperature is decreased to room temperature, the difference in shape is generated due to the difference in thermal shrinkage caused by the difference in temperature. In addition, since a warm steel strip is rolled, as a rolling length to be continuously rolled is increased, a work roll is thermally expanded, and as a result, it is disadvantageously difficult to control the shape of a steel sheet.
In addition, in the method for manufacturing a steel strip disclosed in Japanese Unexamined Patent Application Publication No. 2006-7233 (Patent Document 2), work rolls having a center-line averaged roughness Ra of 2.0 μm or more are used at a final stand of a tandem cold rolling mill which can impart a high tensile force to a steel strip. However, when cold rolling is performed using work rolls having an Ra of 2.0 μm or more, the friction coefficient increases, and as a result, the rolling load unfavorably increases. Furthermore, according to this method, a reduction amount of 8 μm or more is imparted to a steel strip: however, when the reduction is performed at a high stress by the high roughness work rolls as described above, sliding occurs between the steel strip and the work rolls while protuberances thereof stick in the steel strip, and hence, a wear volume of the work roll surface increases. When the center-line averaged roughness Ra is decreased by the wear, a sufficient surface roughness transcription cannot be performed, and as a result, roll exchange must be frequently performed.